Extreme Framerate Loss (Game Run Speed)

[Lagtorio]

I have a 40-ish hour game where I've started upgrading belts and sending trains out for resources. The game has been running full speed, fantastically smooth until last night. I was on a biter killing spree all (real life) day, scouting areas and just getting general revenge. When rounding up 1-200 biters I would experience some occasional choppy frames, which I expect on my GTX 260, but overall great performance. Suddenly, mid-fight, the frames dropped to super-low. I'm guessing 5-10. It became unplayable. I reloaded an autosave, and it ran perfectly. This time I ran back to base and the same thing happened. I've tested with different graphics settings and even just standing still, it always lags like crazy. I checked my entity and sprite count, and it runs fine with 1200-1500 of each, lags when it is down to 300 of each, so it doesn't seem to correspond to a rendering issue. Vsynch didn't help. CPU usage hovers around 60% during this lag.

The best I can hope for is some help here (I haven't seen any similar resolved issues), or let my map run for a few hours and let bodies despawn, then clean up the many thousands of alien artifacts.

TLDR: Crazy and sudden framerate lag not corresponding to anything in particular, aside from 6 hours of recent exploration and battles.

[kinnom]

1: Could you upload the save?
2: Did the game get choppy or did it slow down?

[Loewchen]

Additionally post a screenshot with show_time_used_percent and show_fps activated in the debug settings (F4 > F5) when the issue occurs and post your log file (see my signature).